The Oneness and the Uniquness of the Divine: Rambam's Second Principle of Emuna:
One of the most difficult principles to understand is the oneness of G-d. Rambam defines this to mean that He who is the cause of all things created is one. What is this one? The Ramban explains that the divine cannot be changed because change means addition. If you need to add onto something, that means that it's original form is lacking. This principle of faith means that we understand that G-d is the creator of the world and is not lacking.